Systems and methods for the reduction of peak to average signal levels of multi-bearer single-carrier and multi-carrier waveforms

ABSTRACT

The present invention is related to methods and apparatus that can advantageously reduce a peak to average signal level exhibited by single or by multicarrier multibearer waveforms. Embodiments of the invention further advantageously can manipulate the statistics of the waveform without expanding the spectral bandwidth of the allocated channels. Embodiments of the invention can be applied to either multiple carrier or single carrier systems to constrain an output signal within predetermined peak to average bounds. Advantageously, the techniques can be used to enhance the utilization of existing multicarrier RF transmitters, including those found in third generation cellular base stations. However, the peak to average power level managing techniques disclosed herein can apply to any band-limited communication system and any type of modulation. The techniques can apply to multiple signals and can apply to a wide variety of modulation schemes or combinations thereof.

RELATED APPLICATION

[0001] This application claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Application No. 60/220,018, filed Jul. 21, 2000, theentirety of which is hereby incorporated by reference.

[0002] A co-pending patent application entitled “SYSTEMS AND METHODS FORTHE DYNAMIC RANGE COMPRESSION OF MULTI-BEARER SINGLE-CARRIER ANDMULTI-CARRIER WAVEFORMS,” with Attorney Docket No. DATUMTE.009A,commonly owned and filed on the same day as the present application, ishereby incorporated herein in its entirety by reference thereto.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention generally relates to electronics. Inparticular, the present invention relates to communications systems.

[0005] 2. Description of the Related Art

[0006] The rapid commoditization of the cellular, personal communicationservice (PCS) and wireless industries has resulted in the emergence ofnew digital radio standards, which support the emergence of high userbandwidth requirements. For example, third generation (3G) digitalwide-band code division multiple access (W-CDMA) and Enhanced Data GSM(Group System for Mobile Communications) Environment (EDGE) airinterface standards exploit signal processing techniques that cangenerate radio and baseband waveforms with a relatively high peak powerto average power ratio.

[0007] The signals amplified by a wireless base station include multiplesignals, which are combined to a multi-bearer waveform. The number ofvoice and data connections represented within the multi-bearer waveformcan vary randomly and vary over time. Occasionally, the informationsources that are combined to form the multi-bearer waveform can co-alignand generate a relatively large instantaneous signal peak or crest. Inone example, the relatively large instantaneous signal peak is about 10times higher in power than a nominal or average output level.

[0008] In practice, the alignment that generates a relatively largeinstantaneous signal peak occurs with a relatively low probability.Despite the relatively low probability, however, the dynamic range ofthe entire signal processing chain of a base station should besufficient to handle the large instantaneous signal peak in order totransmit the signal without error.

[0009] One conventional approach is to design the base station toaccommodate the relatively rare, but large, signal peak. As a result,the base station is significantly overdesigned, which results in asignificant increase to the cost of the base station. In particular, thecost and the size of the radio frequency (RF) amplifier of the basestation are deleteriously affected. For example, such an approachdisadvantageously lowers the efficiency of the RF amplifier, as a higherpowered RF amplifier will waste significantly larger amounts of powerfor biases and the like. Further, the extra power dissipation iscorrespondingly dissipated with larger and more costly heat managementtechniques.

[0010] In addition, the relatively large dynamic range imposed upon thebase station by the relatively large signal peak typically requires thatthe upconversion circuitry, the digital to analog converters, thedigital signal processing circuits, and the like also accommodate therelatively large dynamic range.

[0011] In another conventional approach, the signal waveform is hardlimited to reduce the dynamic range of the relatively rare signal peaks.This allows a relatively lower power RF transmitter to be used totransmit the signal, which allows the RF transmitter to operate withrelatively larger efficiency. However, conventional hard limitingtechniques are impractical because hard limiting generates distortionenergy, which causes interference in adjacent channels.

SUMMARY OF THE INVENTION

[0012] Embodiments of the present invention include apparatus andmethods that overcome the disadvantages of the prior art by manipulatinga multibearer waveform, which can include single carrier or multiplecarrier waveforms that reduce the peak to average ratio of themultibearer waveform. Advantageously, embodiments of the presentinvention allow radio frequency (RF) base stations to be more efficient,compact, and lower in cost than conventional base stations.

[0013] Embodiments of the invention permit significant reduction to thecost to provision digital and analog signal processing chains incommunication systems. Embodiments of the invention may be applied to avariety of communications systems including both wire and wirelesscommunications systems such as cellular, personal communications service(PCS), local multipoint distribution systems (LMDS), and satellitesystems.

[0014] One embodiment of the invention includes a waveshaping circuitthat digitally modifies data in a data stream to decrease the amplitudeof signal peaks in a waveform such that an available power of a radiofrequency power amplifier can be efficiently used while preserving thespectral integrity waveform. The waveshaping circuit includes apreconditioning circuit, a pulse generator, a delay circuit and asumming circuit.

[0015] The preconditioning circuit receives an input symbol stream andcompares data in the input symbol stream to a first reference. Thepreconditioning circuit modifies the data in the input symbol stream byapplying a first impulse to the input symbol stream selected to at leastpartially reduce the magnitude of a signal peak in the first waveformwhen the input symbol stream exceeds the first reference. Thepreconditioning circuit further provides the modified symbol stream asan input to a pulse-shaping filter, which maps the modified symbolstream to a baseband stream. The pulse-shaping filter is configured toprovide the baseband stream to a mixer, which upconverts the basebandstream by multiplication with an oscillator signal from an oscillator toan upconverted signal. A plurality of preconditioning circuits can becombined to process a plurality of input symbol streams.

[0016] The pulse generator receives the upconverted signal and receivesphase information from the oscillator. The pulse generator generates aband-limited pulse, such as a Gaussian pulse, a Square Root RaisedCosine (SRRC) pulse, a Raised Cosine (RC) pulse, a Sinc pulse, and thelike, when the pulse generator detects that the upconverted signal has asignal crest above a predetermined threshold. The band-limited pulse issubstantially limited to a frequency band allocated to the input symbolstream.

[0017] The delay circuit is configured to delay the upconverted signalto a delayed upconverted signal, where an amount of delay isapproximately equal to a latency in the pulse generator. The summingcircuit is adapted to sum the band-limited pulse from the pulsegenerator with the delayed upconverted signal from the delay circuit togenerate the first waveform.

[0018] Another embodiment of the invention includes an adaptive controlcircuit that provides parameter updates to a digital waveshapingcircuit. The adaptive control circuit is coupled to at least one inputsymbol stream that is provided as an input to the digital waveshapingcircuit. The adaptive control circuit is further coupled to an outputsample stream, which is generated by the digital waveshaping circuit.The adaptive control circuit includes a reference input adapted toreceive reference information that at least partially controls theparameter updates generated by the adaptive control circuit. Theadaptive control circuit further includes an input monitoring circuitand a receiver circuit that respectively monitor input symbol streamsprovided as inputs to the digital waveshaping circuit and output samplestreams from the digital waveshaping circuit, e.g., a composite output.The adaptive control further includes a parameter update circuit adaptedto calculate and to provide updated parameters to the digitalwaveshaping circuit based on the reference input, a monitored portion ofthe at least one input symbol stream, and the output sample stream.

[0019] Another embodiment of the invention includes a preconditioningcircuit that reduces an amplitude of a signal peak in an input symbolstream in real time. An output of the preconditioning circuit is appliedas an input to a pulse-shaping filter. The preconditioning circuitincludes a comparator, a pseudo random sequence generator, a weightgenerator, a first delay circuit, a multiplier circuit, a second delaycircuit,

[0020] The comparator compares a symbol from the input symbol stream toa reference level and is configured to generate a correction vector whenthe symbol exceeds the reference level. The pseudo random sequencegenerator generates a pseudo random noise sequence. The weight generatorprovides a weight factor based on the correction vector and the receivedpseudo random noise sequence. The first delay circuit delays an impulsefrom the pseudo random sequence generator by a time approximately equalto a latency in the weight generator.

[0021] The multiplier circuit multiplies an impulse from the first delaycircuit with a corresponding weight factor from the weight generator inorder to select the impulse from the pseudo random noise sequence and toscale the selected impulse.

[0022] The second delay circuit delays the input symbol stream by a timeapproximately equal to a latency in the comparator, the weightgenerator, and the multiplier circuit. The summing circuit sums theimpulse selected and scaled by the multiplier circuit with the inputsymbol stream from the second delay circuit to generate the output ofthe preconditioning circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] These and other features of the invention will now be describedwith reference to the drawings summarized below. These drawings and theassociated description are provided to illustrate preferred embodimentsof the invention and are not intended to limit the scope of theinvention.

[0024]FIG. 1 illustrates a waveshaping circuit according to oneembodiment of the present invention.

[0025]FIG. 2 illustrates a complementary cumulative distributionfunction (CCDF) curve for an intrinsic W-CDMA multicarrier signal.

[0026]FIG. 3 illustrates a multi-carrier waveshaping circuit accordingto one embodiment of the present invention.

[0027]FIG. 4 illustrates a waveshaping circuit according to anembodiment of the present invention that adaptively modifies thewaveshaping processing to fit predetermined criteria.

[0028]FIG. 5 illustrates a preconditioning circuit according to anembodiment of the present invention.

[0029] FIGS. 6A-E illustrate an example of the operation of thepreconditioning circuit shown in FIG. 5.

[0030]FIG. 7 graphically represents limiting with a relatively softsignal level threshold and limiting with a relatively hard signal levelthreshold.

[0031]FIG. 8 illustrates another preconditioning circuit according to anembodiment of the present invention.

[0032]FIG. 9 illustrates a waveshaping circuit according to anembodiment of the present invention.

[0033]FIG. 10 consists of FIGS. 10A and 10B and illustrates amulticarrier de-cresting circuit according to an embodiment of thepresent invention.

[0034] FIGS. 11A-E illustrate an example of the operation of themulticarrier de-cresting circuit shown in FIG. 10.

[0035] FIGS. 12A-C are power spectral density (PSD) plots of de-crestingwith a single Gaussian pulse.

[0036] FIGS. 13A-E illustrate de-cresting with multiple Gaussian pulses.

[0037]FIGS. 14A and 14B illustrate the results of a complementaryfrequency domain analysis of a multicarrier de-cresting circuit.

[0038]FIG. 15 illustrates one embodiment of a de-cresting pulsegeneration circuit.

[0039]FIG. 16 illustrates a pulse-shaping filter according to anembodiment of the present invention.

[0040]FIG. 17 consists of FIGS. 17A and 17B and illustrates aphase-modulating waveshaping circuit according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0041] Although this invention will be described in terms of certainpreferred embodiments, other embodiments that are apparent to those ofordinary skill in the art, including embodiments which do not provideall of the benefits and features set forth herein, are also within thescope of this invention. Accordingly, the scope of the present inventionis defined only by reference to the appended claims.

[0042]FIG. 1 illustrates a waveshaping circuit 100 according to oneembodiment of the present invention. A waveshaping circuit can beadapted to shape either single data streams or multiple input streamswith multiple baseband signals. The waveshaping circuit 100 shown inFIG. 1 is adapted to shape a single input data stream to a single shapedoutput data stream. Other embodiments that are adapted to shape and tocombine multiple input signals to a shaped output data stream aredescribed later in connection with FIGS. 3, 4, 9, 10, 15, 16, and 17.

[0043] An input symbol stream 102 is applied as an input to thewaveshaping circuit 100. The input symbol stream 102 can include datafor cellular telephone communications, data communications, and thelike. The waveshaping circuit 100 generates an output sample stream 104as an output. Advantageously, the output of the waveshaping circuit 100has a lower dynamic range than the input symbol stream 102. The lowerdynamic range of the output sample stream 104 allows a base station toprocess and to amplify the output sample stream 104 with lower power andlower dynamic range components.

[0044] The waveshaping circuit 100 includes a preconditioning stage 106,a pulse-shaping and frequency translating circuit 108, and apost-conditioning circuit 110. The waveshaping circuit 100 can replacean upconversion circuit or portions of the waveshaping circuit 100 canbe used to supplement existing upconversion circuits.

[0045] The preconditioning stage 106 includes a preconditioning circuit112. In alternate embodiments, where multiple input baseband signals areshaped and combined, the preconditioning stage 106 can include multiplepreconditioning circuits. The preconditioning circuit 112 appliesnonlinear processing to the input symbol stream 102 on a symbol bysymbol basis. In one embodiment, the preconditioning circuit 112 appliesa soft nonlinear compression function, which severely compressesrelatively extensive signal peaks and compresses relatively modestsignal peaks into a predefined signal range. The output of thepreconditioning circuit 112 is provided as an input to the pulse-shapingand frequency translating circuit 108. At this point in the data flow,bandwidth expansion is not a concern since the output of thepreconditioning circuit 112 exhibits a white spectral characteristic.Further details of the preconditioning circuit 112 are described laterin connection with FIGS. 5, 6, 7, and 8.

[0046] The illustrated pulse-shaping and frequency translating circuit108 includes a pulse-shaping filter 114, a digital numericallycontrolled oscillator (NCO) 116, and a mixer 118. The pulse-shapingfilter 114 maps the source bits of the output of the preconditioningcircuit 112 to a baseband pulse. The output of the pulse-shaping filter114 and an output of the digital NCO 116 are applied as inputs to themixer 118. In one embodiment of the waveshaping circuit 100, thepulse-shaping and frequency translating circuit 108 is implemented withconventional components.

[0047] In a conventional base station without waveshaping, a sequence ofinput modulation symbols is streamed into a pulse-shaping filter and toa frequency upconversion circuit. The modulation symbols usually exhibita white frequency spectral density and it is not until the symbol rateis stepped up to the higher sample stream rate by the pulse-shapingfilter that the new modulation sample stream is band-limited by theactions of the filter. The baseband sample stream output of thepulse-shaping filter can be shifted to a new digital carrier frequencyby multiplication with the output of the digital NCO. The input symbolstream 102 often is a composite of many symbol streams drawn from anumber of active voice and data users. Consequently, on occasion, thesesymbol streams linearly (vectorially) add up to a relatively largesignal peak when relatively many users simultaneously transmit a similaror identical modulation symbol.

[0048] The mere preconditioning of the input symbol stream 102 by thepreconditioning circuit 112 does not adequately reduce peaks in theoutput of the mixer 118 due to Gibbs-type phenomena in the pulse-shapingfilter 114. The Gibbs-type phenomena re-introduces signal peaks to thesignal stream as a natural consequence of filtering.

[0049] In order to compensate for the signal peaks from thepulse-shaping filter 114, the waveshaping circuit 100 includes thepost-conditioning circuit 110. The post-conditioning circuit 110includes a pulse generator 120 and a summing circuit 122. The pulsegenerator 120 detects signal peaks and introduces via the summingcircuit 122 a band-limited Gaussian pulse that destructively interfereswith peaks in the output of the mixer 118 to reduce the peaks in theoutput sample stream 104. Although the destructive interference cantemporarily undermine the waveform integrity of the output sample stream104, the post-conditioning circuit 110 advantageously limits the upperpeak values of the output sample stream 104 to a relatively precisedynamic range.

[0050] This transitory degradation in the integrity of the output samplestream 104 is tolerable, particularly in CDMA systems, because theintroduced error energy is not de-spread in the signal recoveryprocessing undertaken by the receiver. In one embodiment, the pulsegenerator 120 generates a Gaussian pulse or a family of Gaussian pulsesto destructively interfere with the signal peaks in the output of themixer 118. Advantageously, the error energy of a Gaussian pulse orfamily of Gaussian pulses is equally spread among W-CDMA spreadingcodes. In addition to their spectral characteristic, Gaussian pulses canbe generated relatively easily and with relatively low latency. In otherembodiments, the pulse generator 120 uses other types of band-limitedpulse shapes such as Blackman pulses, Hamming pulses, Square Root RaisedCosine (SRRC) pulses, Raised Cosine (RC) pulses, Sinc pulses and thelike to destructively interfere with and reduce the signal peaks.Further details of the post-conditioning circuit 110 are described laterin connection with FIGS. 10 to 17.

[0051]FIG. 2 illustrates a complementary cumulative distributionfunction (CCDF) curve for an intrinsic W-CDMA multicarrier signal. TheW-CDMA multicarrier signal is a multi-bearer waveform that includes atime variant random number of data and voice connections which, onrelatively rare occasions, can co-align and generate a relatively largeinstantaneous signal peak. Although the relatively high amplitude signalpeaks are relatively rare, the probability of the occurrence of therelatively high amplitude signal peaks is non-zero and should beaccommodated by RF transmitters, base stations, and the like.

[0052] A horizontal axis 202 indicates output power relative to anaverage or mean power at 0 decibels (dB). A vertical axis 204 indicatesthe inverse probability (1-P) of the CCDF curve. The curves in FIG. 2illustrate an example of the effects of peak power reduction by thedestructive interference of a waveshaping circuit according to anembodiment of the present invention.

[0053] A first curve 206 corresponds to a typical, i.e., withoutwaveshaping processing, CCDF curve with 10 dB of input back-off (ibo)for an intrinsic W-CDMA multicarrier signal. The first curve 206illustrates that without waveshaping processing, signal levels thatexceed 5 dB above the average signal level occur with a non-zeroprobability. Although the probability of such signal peaks is relativelylow, the entire transmitter, which includes digital processors, analogupconverters, and power amplifiers, should accommodate such signalpeaks.

[0054] A second curve 208 illustrates an example of the effects ofwaveshaping processing according to an embodiment of the presentinvention. The second curve 208 corresponds to a CCDF curve, whereoutput signal peaks have been reduced through destructive interferenceby a waveshaping circuit to limit the signal peaks to a selectedthreshold. In the second curve 208, the selected threshold is about 5 dBabove the mean power. The selected threshold can be varied to correspondto a broad range of values. In one embodiment, the selected threshold isfixed in a waveform shaping circuit. In another embodiment, a waveformshaping circuit monitors the incoming data sequences and adaptivelyadjusts the circuit's behavior to match with predetermined criteria. Thereduction in signal peaks provided by embodiments of the presentinvention advantageously allows signals to be transmitted with moreefficiency and with lower power and lower cost RF amplifiers.

[0055]FIG. 3 illustrates a multi-carrier waveshaping circuit 300according to one embodiment of the present invention, where themulti-carrier waveshaping circuit 300 is adapted to reduce relativelyhigh amplitude signal peaks in a multi-carrier W-CDMA application. Itwill be understood by one of ordinary skill in the art that the numberof carriers can vary over a broad range. The illustrated multi-carrierwaveshaping circuit 300 of FIG. 3 is shown with 3 carriers.

[0056] The multi-carrier waveshaping circuit 300 receives a first inputsymbol stream 302, a second input symbol stream 304 and a third inputsymbol stream 306 as inputs. The multi-carrier waveshaping circuit 300generates an output sample stream 308 by pulse-shaping, upconverting,combining, and waveshaping the input symbol streams.

[0057] The multi-carrier waveshaping circuit 300 includes a firstpreconditioning circuit 310, a second preconditioning circuit 312, athird preconditioning circuit 314, a first pulse-shaping filter 316, asecond pulse-shaping filter 318, a third pulse-shaping filter 320, afirst mixer 322, a second mixer 324, a third mixer 326, a first digitalnumerically controlled oscillator (NCO) 328, a second digital NCO 330, athird digital NCO 332, a post-conditioning pulse generator 348, a firstsumming circuit 350, a delay circuit 352, and a second summing circuit354.

[0058] The first preconditioning circuit 310, the second preconditioningcircuit 312, and the third preconditioning circuit 314 receive as inputsand process the first input symbol stream 302, the second input symbolstream 304 and the third input symbol stream 306, respectively, suchthat the peak to average ratio of each independent baseband inputchannel stream of modulation symbols is constrained within an initiallevel. One embodiment of a preconditioning circuit according to thepresent invention is described in greater detail later in connectionwith FIGS. 5 and 8.

[0059] The outputs of the first preconditioning circuit 310, the secondpreconditioning circuit 312, and the third preconditioning circuit 314,are applied as inputs to the first pulse-shaping filter 316, the secondpulse-shaping filter 318, and the third pulse-shaping filter 320,respectively, which map the inputs to baseband symbol streams.

[0060] The baseband symbol streams are applied as inputs to the firstmixer 322, the second mixer 324, and the third mixer 326. The firstmixer 322, the second mixer 324, and the third mixer 326 mix the symbolstreams with a first output 340, a second output 342, and a third output344 of the first digital NCO 328, the second digital NCO 330, and thethird digital NCO 332, respectively, to upconvert and to producemultiple streams of modulated channels. An output 334 of the first mixer322, an output 336 of the second mixer 324, and an output 338 of thethird mixer 326 are combined to a composite signal by the first summingcircuit 350. In addition, the outputs 334, 336, 338 constructivelyinterfere and destructively interfere with each other when combined. Theconstructive interference and the destructive interference can occureven where the signals that are combined are individuallypre-compensated to limit high-amplitude signal peaks. As a result, thecomposite signal exhibits an even greater dynamic range with asignificantly greater peak to average power ratio than a singlemodulated channel.

[0061] Embodiments of the present invention advantageously compensatefor the relatively high-amplitude signal peaks in composite signalscaused by constructive interference. In addition, embodiments of thepresent invention compensate for the relatively high-amplitude signalpeaks with relatively little, if any, injection of signal energy toadjacent channel allocations. One embodiment that further advantageouslydetects destructive interference to at least partially disable thepre-compensation and the post-compensation applied to the input signalsand to the composite signal is described later in connection with FIG.9.

[0062] The post-conditioning pulse generator 348 compensates for therelatively high-amplitude signal peaks in the composite signal bygenerating multiple Gaussian pulses, which are selected to destructivelyinterfere with relatively high-amplitude signal peaks in the compositesignal. The post-conditioning pulse generator 348 receives as inputs theoutputs 334, 336, 338 and analyzes the phase, frequency and amplitude ofeach respective channel carrier stream. This information permits theGaussian pulse generator control to independently weigh a family ofGaussian pulses and to generate individual Gaussian pulses for eachchannel carrier stream, where each pulse is centered at the respectivecarrier frequency with a phase and amplitude selected to proportionallycancel the particular channel's contribution to the instantaneouscomposite signal's peak. The approach of utilizing multiple pulses isadvantageous because signal energy is not injected into non-utilizedadjacent channel allocations. Injection of signal energy to non-utilizedadjacent channel allocations can undesirably interfere with othertransmitters and systems. Further details of the post-conditioning pulsegenerator 348 are described later in connection with FIGS. 10-17.

[0063] The family of Gaussian pulses generated by the post-conditioningpulse generator 348 is applied as an input to the second summing circuit354. The second summing circuit 354 sums the family of Gaussian pulseswith an output of the delay circuit 352. The delay circuit 352 delaysthe composite signal from the first summing circuit 350 to align thecomposite signal with the Gaussian pulses generated by thepost-conditioning pulse generator 348. In one embodiment, the delaycircuit 352 delays the composite signal by the latency time associatedwith the post-conditioning pulse generator 348 minus the latency timeassociated with the first summing circuit 350. The delay circuit 352 canbe implemented with cascaded flip-flops, delay lines, and the like. Thesecond summing circuit 354 generates the output sample stream 308 as anoutput.

[0064] Waveshaping according to one embodiment of the present inventionincludes three processes: input preconditioning, pulse-shaping, andpost-conditioning de-cresting. Although each process can be configuredto operate independently within a waveshaping circuit, the operatingparameters for each process are preferably selected to complement eachother so that the waveshaping circuit as a whole functions optimally. Inone embodiment, the operating parameters are selected a priori andremain static. In another embodiment, a global de-cresting controlselects operating parameters adaptively and can adjust the operatingparameters dynamically.

[0065]FIG. 4 illustrates a waveshaping circuit 400 according to anembodiment of the present invention that adaptively modifies thewaveshaping processing to fit predetermined criteria. It will beunderstood by one of ordinary skill in the art that the number ofindividual input symbol streams processed by the waveshaping circuit 400can vary over a broad range. The waveshaping circuit 400 shown in FIG. 4is configured to process three such input symbol streams, which are afirst input symbol stream 402, a second input symbol stream 404, and athird input symbol stream 406. As an output, the waveshaping circuit 400generates an output sample stream 408.

[0066] The output sample stream 408 is advantageously monitored by ade-cresting control 416, which calculates and provides updates for thewaveshaping circuit 400 to allow the waveshaping circuit to adapt thewaveshaping processing to the input symbol stream. The de-crestingcontrol 416 also monitors the first input symbol stream 402, the secondinput symbol stream 404, and the third input symbol stream 406. Inaddition, the de-cresting control 416 receives a reference information418.

[0067] In response to the monitored input symbol streams 402, 404, 406,the monitored output sample stream 408, and the reference information418, the de-cresting control 416 generates and provides parameterupdates to the first preconditioning circuit 410, to the secondpreconditioning circuit 412, to the third preconditioning circuit 414,and to the post-conditioning pulse generator 428. The parameter updatescan include updates to coefficients used in digital filters, such as afinite impulse response (FIR) filter.

[0068] The first preconditioning circuit 410, the second preconditioningcircuit 412, the third preconditioning circuit 414, and thepost-conditioning pulse generator 428 shown in FIG. 4 are similar to thefirst preconditioning circuit 310, the second preconditioning circuit312, the third preconditioning circuit 314, and the post-conditioningpulse generator 348 described earlier in connection with FIG. 3. Furtherdetails of a preconditioning circuit are described later in connectionwith FIGS. 5, 7, and 8.

[0069] In one embodiment, the reference information 418 controls anamount of dynamic range compression by the waveshaping circuit 400. Thereference information 418 can also be used to control a relative“hardness” or relative “softness” of limiting as described later inconnection with FIG. 7. The de-cresting control 416 permits the overallperformance of the waveshaping circuit 400 to be monitored and permitsadjustments to be made to the parameters of individual, multiple or allof the sub-components of the waveshaping circuit 400. For example, thede-cresting control 416 can be used to adapt the processing of awaveshaping circuit to RF transmitters with a broad range of outputpower.

[0070] The de-cresting control 416 does not have to provide parameterupdates in real time. In one embodiment, the de-cresting control 416 isimplemented by firmware in a general purpose DSP or by a general-purposemicroprocessor or microcontroller. In one embodiment, the generalpurpose DSP or the general purpose microprocessor resides in an externalcircuit and interfaces to the first preconditioning circuit 410, to thesecond preconditioning circuit 412, to the third preconditioning circuit414, and to the post-conditioning pulse generator 428. In anotherembodiment, the de-cresting control 416, together with other componentsof the waveshaping circuit 400, is implemented with an applicationspecific integrated circuit (ASIC) or with a field programmable gatearray (FPGA).

[0071]FIG. 5 illustrates a preconditioning circuit 500 according to anembodiment of the present invention. The preconditioning circuit 500exploits the white spectral properties of an input symbol stream 502.The input symbol stream 502 includes a sequence of modulation symbolimpulses or rectangular pulses and occupies a relatively wide frequencyspectrum prior to pulse shaping by a pulse-shaping circuit. Thesubsequent pulse-shaping circuit filters a modified symbol stream 504and provides the overall spectral shaping to apply the specifiedbandwidth constraints.

[0072] One embodiment of the preconditioning circuit 500 advantageouslyexploits the pulse shaping by the pulse-shaping circuit to modify theoverall signal characteristics of the input symbol stream 502 byapplication of both linear and non-linear signal processing techniques.The spectral expansion induced by non-linear signal processing is laterremoved by the pulse-shaping circuit. In one embodiment, a subsequentpost-conditioning circuit, such as a post-conditioning pulse generator,is not permitted to process in a manner that would expand the spectrumoccupied by the processed signal. One embodiment of thepost-conditioning circuit accordingly processes the applied signal withlinear signal processing. However, exceptions are conceivable.

[0073] One embodiment of the preconditioning circuit 500 uses a pseudorandom sequence of pulses that is weighted to destructively interferewith selected pulses of the input symbol stream 502 and to select anamount of destructive interference.

[0074] With reference to FIG. 5, the illustrated preconditioning circuit500 includes a comparator 506, a first delay circuit 508, a weightgenerator 512, a pseudo random sequence generator 514, a second delaycircuit 516, a multiplier 518, and a summing circuit 520. Furtheroperational details of the preconditioning circuit 500 are alsodescribed later in connection with FIGS. 6A-E.

[0075] The input symbol stream 502 is applied as an input to thecomparator 506 and to the first delay circuit 508. The comparator 506detects the level of the instantaneous magnitude of the input symbolstream 502 and compares the level to a reference level information 510to determine whether to apply signal preconditioning to the input symbolstream. The reference level information 510 can be used to indicate athreshold or a limit to the magnitude and/or phase of a signal peak. Inone embodiment, the reference level information 510 is staticallypredetermined a priori and hard coded into the preconditioning circuit500. In another embodiment, the reference level information 510 isadaptively provided by the de-cresting control, which can be an internalfunction or circuit of the waveshaping circuit or provided by a functionor circuit external to the waveshaping circuit. When the comparisonindicates that signal preconditioning is to be applied, the comparator506 applies a correction vector as an input to the weight generator 512.

[0076] The weight generator 512 receives the correction vector from thecomparator 506 and a pseudo random sequence from the pseudo randomsequence generator 514. In response to the correction vector and thepseudo random sequence, the weight generator 512 computes a weightfactor, which is applied as an input to the multiplier 518. The weightfactor, when applied to the pseudo random sequence, generates theappropriate correction vector that is linearly added to a delayedversion of the input symbol stream 502 to destructively interfere withrelatively high-amplitude signal peaks in the input symbol stream 502.In one embodiment, the weight factor is a scalar quantity that dependson a complex value of the input symbol stream 502 and a complex value ofthe pseudo random sequence.

[0077] The second delay circuit 516 delays the pseudo random sequencefrom the pseudo random sequence generator 514 to align the pseudo randomsequence with the weight factor from the weight generator. The weightfactor and the delayed pseudo random sequence are multiplied together bythe multiplier 518 to generate the correction impulses.

[0078] The input symbol stream 502 is delayed by the first delay circuit508. The first delay circuit 508 is configured to delay the input symbolstream 502 such that the input symbol stream 502 aligns with thecorrection impulses. In one embodiment, the first delay circuit 508delays the input symbol stream 502 by an amount of time approximatelyequal to the latency of the comparator 506, the weight generator 512,and the multiplier 518. The delays provide the preconditioning circuit500 with time to determine whether a modifying impulse or pulse is to beintroduced into the data flow in order to reduce a relatively highsignal peak or crest in the data sequence and to determine an amount ofa reduction in the magnitude and/or phase of the crest.

[0079] The delayed input symbol stream from the first delay circuit 508is linearly summed by the summing circuit 520 with the correctionimpulses from the multiplier 518. The linear superposition of thesumming circuit 520 generates the modified symbol stream 504 as anoutput. The relatively high signal peaks in the input symbol stream 502are reduced in the modified symbol stream 504 by destructiveinterference of the input symbol stream 502 with the correctionimpulses.

[0080] Advantageously, the illustrated preconditioning circuit 500 canproduce both phase variations and amplitude variations in the inputsymbol stream 502 to de-crest the input symbol stream 502. The abilityto provide a phase variation finds particular utility in multi-carrierapplications, as will be described in connection with FIGS. 3, 4, 9, and10.

[0081] FIGS. 6A-E illustrate an example of the operation of thepreconditioning circuit 500 illustrated in FIG. 5. For clarity, theexample shown in FIGS. 6A-E is drawn with the input symbol stream 502and the pseudo random sequence represented as scalar quantities. It willbe understood by one of ordinary skill in the art that both the inputsymbol stream 502 and the pseudo random sequence are generally complexquantities with both magnitude and phase. Also for clarity, the exampleshown in FIGS. 6A-E does not show the delay in the first delay circuit508 and in the second delay circuit 516.

[0082] In FIGS. 6A-E, a plurality of horizontal axes 602, 604, 606, 608,610 indicate time. FIG. 6A illustrates an example of the input symbolstream 502, which is applied as an input to the preconditioning circuit500. Dashed lines 612, 614 indicate a predetermined threshold level. Forexample, the predetermined threshold level can correspond to a peakoutput power level of an associated RF transmitter. In the example, fourevents 616, 618, 620, 622 exceed the predetermined threshold level.

[0083]FIG. 6B illustrates a time aligned pseudo random sequence ofconstant amplitude signal pulses from the pseudo random sequencegenerator 514. FIG. 6C illustrates a sequence of weight factors that arecalculated by the weight generator 512. The weight factors are appliedto the pseudo random sequence to generate the correction impulses. FIG.6D illustrates a sequence of the correction impulses for thepreconditioning circuit 500.

[0084]FIG. 6E illustrates the modified symbol stream 504. The modifiedsymbol stream 504 is the time-aligned linear superposition of the inputsymbol stream 502 with the correction impulses. The correction impulsesdestructively interfere with the four events 616, 618, 620, 622 shown inFIG. 6A so that an output level of the modified symbol stream 504 shownin FIG. 6E remains at or below the predetermined threshold level asshown by the dashed lines 612, 614. In one embodiment, thepreconditioning circuit 500 applies correction impulses to the inputsymbol stream 502 such that the modified symbol stream 504 does nottransgress beyond a selected signal level threshold.

[0085]FIG. 7 graphically represents limiting with a relatively softsignal level threshold and limiting with a relatively hard signal levelthreshold. A horizontal axis 702 indicates an input level. A verticalaxis 704 indicates an output level.

[0086] A first trace 706 corresponds to limiting with a relatively hardsignal level threshold. In practice, the use of a single hard signallevel threshold is not appropriate because the resulting complementarycumulative distribution function (CCDF) of the signal, as describedearlier in connection with FIG. 2, will not exhibit a smooth transitionbut rather an abrupt or rapid “cliff.” Such an approach often results inan unacceptably high error rate in the downstream receiver.

[0087] The preconditioning circuits according to the present inventionadvantageously overcome the disadvantages of relatively hard signallevel thresholding by employing a nonlinear weighting function thatprovides a varying amount of correction depending upon the magnitude ofthe input data stream. A second trace 708, a third trace 710, and afourth trace 712 represent exemplary transfer functions associated witha relatively soft signal-leveling threshold.

[0088] This approach of soft weighting eliminates the rapid onset of a“cliff” in the CCDF and replaces the abrupt cliff with a relatively softregion in which the probability of a signal level exceeding apredetermined signal level is significantly less than that exhibited bythe intrinsic input symbol stream. At relatively high signal levels, thenon-linear weighting function approaches a hard threshold, and a delay“cliff” in the signal's CCDF occurs. The soft weighting approach does,however, provide a significant decrease in the level of error energyobserved by the downstream receivers.

[0089] The preconditioning circuit 500 operates by deliberatelymanipulating the amplitude and phase probability density function of theinput signal waveform so that the peak to average of the input signal'simpulse stream is significantly lower than the original input waveform.In practice, any function or non-linear equation that exhibits behaviorwhich incurs desirable changes in the weight calculation can be employedby the preconditioning circuit 500. In one embodiment, the non-linearweighting function is expressed by Equation 1. In addition, thedeliberate insertion of Amplitude Modulation (AM), Phase Modulation(PM), or both can require an alternative function.

[0090] Equation 1 defines a family of soft preconditioning weightingfunctions. Equation 1 includes parameters α and β, which correspond tothe degree of non-linearity invoked. $\begin{matrix}{{V_{m}(t)} = {\frac{\left| {V_{m}(t)} \right|}{\left( {1 + \left( \frac{\left| {V_{m}(t)} \right|}{\beta} \right)^{\alpha}} \right)^{1/\alpha}}e^{j{({\arg {({V_{m}{(t)}})}})}}}} & {{Eq}.\quad 1}\end{matrix}$

[0091] As α increases, the gain of the function increases, which permitsan overall level of preconditioning to be defined. Manipulation of βpermits the rate at which a hard clipping level is set.

[0092]FIG. 8 illustrates another preconditioning circuit 800 accordingto an embodiment of the present invention. The illustratedpreconditioning circuit 800 uses multipliers and coefficients tocalculate a Taylor series expansion of the non-linear weighting functionshown in Equation 1.

[0093] The approximation of the non-linear weighting function by theTaylor series expansion includes at least three engineering compromises:delay latency, power consumption, and precision of the Taylor seriesapproximation. The delay latency of the preconditioning circuit 800increases as a function of the order of the Taylor series expansion,i.e., increases with the number of multiplier stages. The powerconsumption of the preconditioning circuit 800 increases as the numberof multipliers is increased. The weighting function is less closelyapproximated by the Taylor series expansion, where fewer terms of theTaylor series expansion are computed.

[0094] The Taylor series approximation approach uses relativelyextensive delay balancing between each of the signal processing paths toensure that the calculated preconditioning function, represented in FIG.8 as “p,” applies to the appropriate input samples. The illustratedpreconditioning circuit 800 computes the Taylor series expansion to thefourth order. It will be understood by one of ordinary skill in the artthat the preconditioning circuit 800 can be implemented in software aswell as in hardware.

[0095] The illustrated preconditioning circuit 800 includes a magnitudecomputation circuit 802, a first delay circuit 804, a first multiplier806, a second multiplier 808, a third multiplier 810, a second delaycircuit 812, a third delay circuit 814, a fourth delay circuit 816, afifth delay circuit 818, a sixth delay circuit 820, a coefficient bank822, a fourth multiplier 824, a fifth multiplier 826, a sixth multiplier828, a seventh multiplier 830, a summing circuit 832, an eighthmultiplier 834, and a ninth multiplier 836.

[0096] Generally, the input symbol stream is complex, with both anin-phase component and a quadrature-phase component. The in-phasecomponent of the input symbol stream, I_(input), is applied as an inputto the magnitude computation circuit 802 and to the first delay circuit804. The quadrature phase component of the input symbol stream,Q_(input), is applied as an input to the magnitude computation circuit802 and to the first delay circuit 804. The magnitude computationcircuit 802 computes the magnitude of the input symbol stream. In oneembodiment, the computed magnitude corresponds approximately to a sum ofsquares.

[0097] An output of the magnitude computation circuit 802, termed“magnitude,” is applied as an input to the first multiplier 806, thesecond delay circuit 812, and the fourth delay circuit 816. The firstmultiplier 806 multiplies the magnitude by itself to produce a square ofthe magnitude as an output. The output of the first multiplier 806 isapplied as an input to the second multiplier 808 and to the fifth delaycircuit 818.

[0098] The second multiplier 808 receives and multiplies the output ofthe first multiplier 806 and an output of the second delay circuit 812.The second delay circuit 812 delays the magnitude or the output of themagnitude computation circuit 802 by a latency associated with the firstmultiplier 806. The second multiplier 808 multiplies the squaredmagnitude from the first multiplier 806 with the first delayed magnitudefrom the second delay circuit 812 to generate a cubed magnitude.

[0099] The cubed magnitude output of the second multiplier is applied asan input to the third multiplier 810 and to the sixth delay circuit 820.The first delayed magnitude output of the second delay circuit 812 isapplied as an input to the third delay circuit 814, which generates asecond delayed magnitude. The second delayed magnitude from the thirddelay circuit 814 and the cubed magnitude from the second multiplier 808are provided as inputs to the third multiplier 810. The third multiplier810 generates an output, which corresponds to the magnitude raised tothe fourth power.

[0100] The output of the third multiplier 810 is provided as an input tothe seventh multiplier 830. The output of the third multiplier 810 isdelayed from the magnitude output of the magnitude computation circuit802 by the sum of the latency time of the first multiplier 806, thelatency time of the second multiplier 808, and latency time of the thirdmultiplier 810. The sixth delay circuit 820, the fifth delay circuit818, and the fourth delay circuit 816 delay samples such that Taylorseries expansion terms combined by the summing circuit 832 correspond tothe same sample.

[0101] The sixth delay circuit 820 delays the magnitude cubed output ofthe second multiplier 808 by the latency time of the third multiplier810 to time align the magnitude cubed output with the magnitude to thefourth power of the third multiplier 810.

[0102] The fifth delay circuit 818 delays the magnitude squared outputof the first multiplier 806 by the sum of the latency time of the secondmultiplier 808 and the latency time of the third multiplier 810. Thefifth delay circuit 818 time aligns the magnitude squared output of thefirst multiplier 806 with the magnitude to the fourth power output ofthe third multiplier 810.

[0103] The fourth delay circuit 816 delays the magnitude output of themagnitude computation circuit 802 approximately by the sum of thelatency time of the first multiplier 806, the latency time of the secondmultiplier 808, and the latency time of the third multiplier 810. Itwill be understood by one of ordinary skill in the art that the fourthdelay circuit 816, the fifth delay circuit 818, and the sixth delaycircuit 820 can be placed in the signal path either before or after thefourth multiplier 824, the fifth multiplier 826, and the sixthmultiplier 828, respectively.

[0104] The fourth multiplier 824, the fifth multiplier 826, the sixthmultiplier 828, and the seventh multiplier 830 compute the individualterms of the Taylor series expansion. The coefficient bank 822 storesthe coefficients of the Taylor series expansion. The coefficients areapplied as inputs to the fourth multiplier 824, to the fifth multiplier826, to the sixth multiplier 828, and to the seventh multiplier 830. Theoutputs of the fourth delay circuit 816, the fifth delay circuit 818,the sixth delay circuit 820 and the third multiplier 810 are alsoapplied as inputs to the fourth multiplier 824, the fifth multiplier826, the sixth multiplier 828, and the seventh multiplier 830,respectively. In one embodiment, the latency times of the fourthmultiplier 824, the fifth multiplier 826, the sixth multiplier 828, andthe seventh multiplier 830 are approximately equal.

[0105] The outputs of the fourth multiplier 824, the fifth multiplier826, the sixth multiplier 828, and the seventh multiplier 830 areprovided as inputs to the summing circuit 832 to compute the Taylorseries expansion of the preconditioning function. The output of thesumming circuit 832 is provided as an input to the eighth multiplier 834and to the ninth multiplier 836. The outputs of the first delay circuit804 are also provided as inputs to the eighth multiplier 834 and to theninth multiplier 836.

[0106] The first delay circuit 804 delays the in-phase component of theinput symbol stream and the quadrature-phase component of the inputsymbol stream to time align the in-phase component and thequadrature-phase component with the corresponding preconditioningfunction as provided by computation of the Taylor series expansion. Inone embodiment, the delay of the first delay circuit 804 isapproximately the sum of the latency time of the magnitude computationcircuit 802, the latency time of the first multiplier 806, the latencytime of the second multiplier 808, the latency time of the thirdmultiplier 810, the latency time of the seventh multiplier 830, and thelatency time of the summing circuit 832.

[0107] The preconditioning circuit 800 illustrated in FIG. 8 can beimplemented in hardware or by software. For example, where the data rateis relatively low, the preconditioning circuit 800 can be implemented bysoftware running on a general-purpose digital signal processor (DSP) ora microprocessor. In a relatively wideband application, thepreconditioning circuit 800 can be fabricated in dedicated hardwarewith, for example, a field programmable gate array (FPGA) or with anapplication specific integrated circuit (ASIC).

[0108]FIG. 9 illustrates another waveshaping circuit 900 according toone embodiment of the present invention. The waveshaping circuit 900receives multiple input symbol streams and advantageously detects whenthe multiple input symbol streams fortuitously destructively interferewith each other such that an amount of preconditioning applied to theindividual input symbol streams can be decreased or eliminated.

[0109] In the multi-carrier waveshaping circuit 300 and the waveshapingcircuit 400 described earlier in connection with FIGS. 3 and 4,respectively, an individual preconditioning circuit independentlyapplies preconditioning to limit a relatively high signal peak in itsrespective input symbol stream. However, where multiple input symbolstreams are eventually combined, such as by the first summing circuit350 described in connection with FIGS. 3 and 4, the multiple inputsymbol streams may on occasion destructively interfere with each other.On these occasions, the preconditioning applied to relatively highsignal peaks in the input symbol streams can be decreased or eliminated,thereby reducing or eliminating the associated injection of error energythat otherwise would have been injected into the composite multicarrierwaveform stream by the preconditioning circuits and thepost-conditioning circuit.

[0110] For illustrative purposes, the waveshaping circuit 900 shown inFIG. 9 processes three input symbol streams. However, it will beunderstood by one of ordinary skill in the art that the number of inputsymbol streams processed by embodiments of the present invention isarbitrary. A broad range of input symbol streams can be processed byembodiments of the present invention.

[0111] The illustrated waveshaping circuit 900 includes the firstpulse-shaping filter 316, the second pulse-shaping filter 318, the thirdpulse-shaping filter 320, the first mixer 322, the second mixer 324, thethird mixer 326, the first digital NCO 328, the second digital NCO 330,the third digital NCO 332, and the first summing circuit 350 describedearlier in connection with FIG. 3. The waveshaping circuit 900 furtherincludes a first preconditioning circuit 910, a second preconditioningcircuit 912, a third preconditioning circuit 914, a crest predictiveweight generator 916, a post-conditioning pulse generator 928, a secondsumming circuit 930, and a delay circuit 932.

[0112] A first input symbol stream 902, a second input symbol stream904, and a third input symbol stream 906 are applied as inputs to thefirst preconditioning circuit 910, the second preconditioning circuit912, the third preconditioning circuit 914, respectively, and to thecrest predictive weight generator 916. The first preconditioning circuit910, the second preconditioning circuit 912, the third preconditioningcircuit 914, respectively, and to the crest predictive weight generator916 can be similar to the preconditioning circuits described inconnection with FIGS. 5 and 8.

[0113] A digital NCO phase information 934, a second digital NCO phaseinformation 936, and a third digital phase information 938 from thefirst digital NCO 328, the second digital NCO 330, and the third digitalNCO 332, respectively, are applied as inputs to the crest predictiveweight generator 916. The phase information allows the crest predictiveweight generator 916 to determine how the input symbol streams willcombine. The crest predictive weight generator 916 can use pulse-shapingfilters to predict how the input symbol streams will combine. In oneembodiment, the length, the latency, or both the latency and the lengthof the pulse-shaping filters of the crest predictive weight generator916 is less than the length, the latency, or both the latency and thelength of the pulse-shaping filters 316, 318, 320.

[0114] The crest predictive weight generator 916 examines the multipleinformation symbol streams and the corresponding phases of the digitalnumerical controlled oscillators to determine or to predict whether arelatively high-level signal crest will subsequently occur in thecombined signal. When the crest predictive weight generator 916 predictsthat a relatively high-amplitude signal crest will occur in the combinedsignal, the crest predictive weight generator 916 provides weight valuesto the pre-conditioning circuits that allow the preconditioning circuitsto individually process their respective input symbol streams to reducethe relatively high amplitude signal peaks. When the crest predictiveweight generator 916 predicts that destructive interference between thesymbol streams themselves will reduce or will eliminate the relativelyhigh-level signal crest, the crest predictive weight generator 916provides weight values to the preconditioning circuits that reduce ordisable the preconditioning applied by the preconditioning circuits.

[0115] The crest predictive weight generator 916 can optionally providean advanced crest occurrence information 940 to the post-conditioningpulse generator 928. The advanced crest occurrence information 940 canadvantageously be used to reduce computation latency in the waveshapingcircuit 900 by allowing the post-conditioning pulse generator 928 toinitiate early production of band-limited pulses, such as Gaussianpulses, which are applied to destructively interfere with a compositesignal output of the delay circuit 932. In other aspects, one embodimentof the post-conditioning pulse generator 928 is similar to thepost-conditioning pulse generator 348 described earlier in connectionwith FIG. 3.

[0116] One embodiment of the crest predictive weight generator 916provides the weight value as a binary value with a first state and asecond state. For example, in the first state, the crest predictiveweight generator 916 allows waveshaping, and in the second state, thecrest predictive weight generator 916 disables waveshaping. The crestpredictive weight generator 916 provides the weight value or values tothe preconditioning circuits and the crest occurrence information to thepost-conditioning circuit in real time and not in non-real time. Bycontrast, the de-cresting control 416 described in connection with FIG.4 can provide parameter updates to preconditioning and topost-conditioning circuits in either real time or in non-real time. Inone embodiment, a waveshaping circuit includes both the crest predictiveweight generator 916 and the de-cresting control 416.

[0117] The advanced crest occurrence information 940 allows the crestpredictive weight generator 916 to notify the post-conditioning pulsegenerator 928 of when the input symbol streams at least partiallydestructively interfere when combined. This allows the post-conditioningpulse generator to correspondingly decrease the magnitude of theband-limited pulse or to eliminate the band-limited pulse that wouldotherwise be applied by the post-conditioning pulse generator 928 to thecomposite signal to reduce relatively high-amplitude signal peaks.

[0118] In one embodiment, the first preconditioning circuit 910, thesecond preconditioning circuit 912, and the third preconditioningcircuit 914 are adapted to receive weight values 920, 922, 924 from thecrest predictive weight generator 916 and are also adapted to modify thepreconditioning according to the received weight values. In oneembodiment, the weight values 920, 922, 924 are the same for eachpreconditioning circuit and can be provided on a single signal line. Inanother embodiment, the weight values 920, 922, 924 are individuallytailored for each preconditioning circuit.

[0119] The preconditioning circuit 500 described in connection with FIG.5 can be modified to be used for the first preconditioning circuit 910,the second preconditioning circuit 912, or the third preconditioningcircuit 914 by allowing the applied weight value provided by the crestpredictive weight generator 916 to vary the weight applied by the weightgenerator 512. In another embodiment, the weight value from the crestpredictive weight generator 916 disables the summation of the inputsymbol stream 502 with the correction impulse by, for example, partiallydisabling the summing circuit 520, disabling the multiplier 518, or byotherwise effectively zeroing the correction impulse.

[0120] The preconditioning circuit 800 described in connection with FIG.8 can also be modified to be used for the first preconditioning circuit910, the second preconditioning circuit 912, and the thirdpreconditioning circuit 914. For example, when the amount ofpreconditioning is decreased, the weight values applied to thepreconditioning circuit 800 can be used to select alternativecoefficients in the coefficient bank 822. The weight values can also beused to decrease a magnitude of the applied preconditioning by, forexample, attenuating the output of the summing circuit 832. Where thepreconditioning is disabled, the weight value can be used to disable aportion of the preconditioning circuit 800, such as the summing circuit832 or the eighth multiplier 834 and the ninth multiplier 836, todisable the preconditioning.

[0121] The waveshaping circuit 900 can further include an additionaldelay circuit to compensate for computational latency in the crestpredictive weight generator 916. In one embodiment, the firstpreconditioning circuit 910, the second preconditioning circuit 912, andthe third preconditioning circuit 914 include the additional delaycircuit.

[0122] In addition to detecting when the input symbol streamsdestructively interfere with each other so that an amount of waveshapingcan be reduced or eliminated, one embodiment of the crest predictiveweight generator 916 advantageously detects when a relatively shorttransitory sequence of impulses or pulses from the information sourcesequentially exhibits similar amplitude and phase levels and wouldotherwise give rise to a relatively large crest.

[0123] Pulse-shaping filters, such as the first pulse-shaping filter316, the second pulse-shaping filter 318, and the third pulse-shapingfilter 320, limit the spectral occupancy of impulse and pulseinformation-bearing data streams in communication systems. A deleteriouscharacteristic of these filters is that the peak to average of the pulseor impulse stream is invariably expanded during the pulse-shapingprocess, often by in excess of 3 dB. These newly introduced signalcrests are generally attributed to Gibbs filter ringing effects.Ordinarily, relatively large crests occur when a relatively shorttransitory sequences of impulses or pulses from the information sourcessequentially exhibit similar amplitude and phase levels. These scenariosmay be advantageously predicted by the crest predictive weight generator916.

[0124] Upon detection of the relatively short transitory sequence ofimpulses or pulses that sequentially exhibit similar amplitude and phaselevels, the crest predictive weight generator 916 selects compensationwith a sequence of corrective vectors rather than compensation with asingle corrective vector. This distributes the introduction of errorenergy over a short sequence of modulation symbols rather than to asingle symbol. In systems that do not exploit code division multipleaccess (CDMA), such as Enhanced Data GSM Environment (EDGE), thedistribution of the error energy is desirable because it mitigatesagainst the impact of error energy upon the downstream receiver'sdetector error rate.

[0125]FIG. 10 illustrates further details of a multicarrier de-crestingcircuit 1000 according to an embodiment of the present invention. Theillustrated multicarrier de-cresting circuit 1000 does not includepre-conditioning of the input symbol streams.

[0126] The multicarrier de-cresting circuit 1000 shown in FIG. 10includes a multiple channel circuit 1002, a de-cresting pulse generationcircuit 1004, and a de-cresting combiner 1006. The multiple channelcircuit 1002 pulse-shapes, upconverts, and combines multiple inputsymbol streams. In one embodiment of the multicarrier de-crestingcircuit 1000, the multiple channel circuit 1002 corresponds to aconventional circuit. Another embodiment of the multicarrier de-crestingcircuit 1000 uses a multiple channel circuit described in greater detaillater in connection with FIG. 16.

[0127] The de-cresting pulse generation circuit 1004 generates carrierwaveforms and generates post-compensation band-limited de-crestingpulses. A pulse generator control 1008 receives and inspects a compositemulticarrier signal M_(c)(t) 1010, individual subcarrier signals (orbaseband equivalents), and digital NCO waveforms. This permits the pulsegenerator control 1008 to determine the requirement for, the totalnumber of, the duration, the frequency, the amplitude and the phase ofband-limited pulses that are to be injected into the transmission datastream to reduce or to eliminate relatively high amplitude peaks in thecomposite multicarrier signal 1010. In one embodiment, the band-limitedpulses are Gaussian pulses that are provided by a bank of generalizedGaussian pulse generators that accept commands from the pulse generatorcontrol 1008 to generate a pulse of a specific duration, phase,amplitude and center frequency. Further details of the de-cresting pulsegeneration circuit 1004 are described later in connection with FIG. 15.Further details of the pulse generator control 1008 are described laterin connection with FIGS. 13A-E.

[0128] The de-cresting combiner 1006 combines the upconverted inputsymbol streams with the post-compensation band-limited de-crestingpulses to remove the relatively high-level signal crests from thecombined input symbol streams. The de-cresting combiner 1006 includes atime delay circuit 1012. The time delay circuit 1012 delays thecomposite multicarrier signal 1010 to a time-delayed compositemulticarrier signal 1016. The delay of the time delay circuit 1012 ismatched to the corresponding delay in the de-cresting pulse generationcircuit 1004 so that a desired amount of destructive interference can bereliably induced. An output of the time delay circuit 1012 is providedas an input to a multi-input summing junction 1014, which provides ade-crested composite multicarrier signal 1018 as the linear sum of thecomposite multicarrier signal 1010, as delayed by the time delay circuit1012, and a collection of band-limited pulses. It will be understood byone of ordinary skill in the art that the band-limited pulses can beindividually applied to the multi-input summing junction 1014 or theband-limited pulses can be combined to a composite pulse stream and thenapplied to the multi-input summing junction 1014.

[0129] In one embodiment, the band-limited pulses are Gaussian pulses.The collection of Gaussian pulses can include zero, one, or multiplepulses depending on the instantaneous magnitude of the compositemulticarrier signal 1010.

[0130] FIGS. 11A-E illustrate an example of the operation of themulticarrier de-cresting circuit 1000 shown in FIG. 10. With referenceto FIGS. 11A-E, horizontal axes 1102, 1104, 1106, 1108, 1110 indicatetime. As shown in FIGS. 11A-E, time increases to the right. FIG. 11Aincludes a first waveform 1112, which corresponds to an illustrativeportion of the composite multicarrier signal 1010. The first waveform1112 further includes a waveform crest 1114, which corresponds to arelatively high-amplitude signal crest in the composite multicarriersignal 1010. Although the average power level of the compositemulticarrier signal 1010 can be relatively low, the waveform crest 1114illustrates that the information sources, which contribute to the inputsymbol streams, can occasionally align and generate a relativelyhigh-amplitude signal peak. For example, a signal peak that is about 10dB above the average power level can occur with a probability of 10⁻⁴.In another example, 14 dB signal peaks can occur with a probability of10⁻⁶.

[0131]FIG. 11B illustrates a second waveform 1116 with a pulse 1118. Thepulse 1118 of the second waveform 1116 corresponds to a band-limitedpulse, such as a Gaussian pulse, which is generated by the de-crestingpulse generation circuit 1004 to destructively interfere with therelatively high-amplitude signal crest in the composite multicarriersignal 1010 as illustrated by the waveform crest 1114.

[0132]FIG. 11C illustrates a third waveform 1120, which corresponds tothe time-delayed composite multicarrier signal 1016. The time delaycircuit 1012 delays the composite multicarrier signal 1010 to thetime-delayed composite multicarrier signal 1016 to compensate for thecomputational latency of the de-cresting pulse generation circuit 1004.This alignment is shown in FIGS. 11B and 11C by the alignment of thedelayed signal crest 1122 with the pulse 1118.

[0133] The band-limited pulse destructively interferes with therelatively high signal peak in the time-delayed composite multicarriersignal 1016. FIG. 11D illustrates a fourth waveform 1124, whichcorresponds to the output of the multi-input summing junction 1014. Thefourth waveform 1124 is thus the linear superposition of the secondwaveform 1116 and the third waveform 1120. In the fourth waveform 1124,a compensated portion 1126 is substantially devoid of the waveform crest1114 by the destructive interference induced by the band-limited pulse.FIG. 11E superimposes the second waveform 1116, the third waveform 1120,and the fourth waveform 1124.

[0134] FIGS. 12A-C illustrate a complementary frequency domain analysisof the multicarrier de-cresting circuit that uses only a single Gaussianpulse to de-crest a composite waveform. FIG. 12A illustrates an exampleof a basic power spectral density plot (PSD) of a composite singlecarrier signal 1202 and a PSD plot of a single Gaussian pulse 1204. FIG.12A also illustrates a resulting output signal power spectral density1206 when the composite single carrier signal 1202 and the singleGaussian pulse 1204 are linearly combined. In one embodiment, themulticarrier de-cresting circuit 1000 expands the PSD only when theGaussian pulse's characteristics expand the signal energy beyond thebasic frequency allocation. Thus, the bandwidth expansion of thecombined signal is readily controlled by controlling the characteristicsof the de-cresting pulse generation circuit 1004 configured to generatea single Gaussian pulse.

[0135]FIG. 12B also illustrates the applicability of a generating asingle Gaussian pulse to reduce a magnitude of a signal crest in amulticarrier application. A trace 1208 corresponds to a basic PSD plotcorresponding to a multicarrier signal crest. A trace 1210 correspondsto a PSD plot of the single Gaussian pulse. A trace 1212 illustrates acomposite PSD of the combination of the multicarrier signal crest withthe single Gaussian pulse.

[0136]FIG. 12C illustrates a disadvantage of generating a singleGaussian pulse to reduce the magnitude of a signal crest in amulticarrier signal. In the example shown in FIG. 12C, one of thechannel streams is dropped either temporarily or permanently from thecomposite multicarrier signal 1010. A trace 1214 corresponds to a basicPSD of the multicarrier signal crest with a channel stream dropped. Atrace 1216 corresponds to a PSD plot of the single Gaussian pulse. Atrace 1218 illustrates a composite PSD of the combination of the singleGaussian pulse and the multicarrier signal crest with the channel streamdropped. As shown in FIG. 12C, energy from the Gaussian pulse increasesthe residual energy level within the unoccupied channel allocation. Theincrease in residual energy in the unoccupied channel is relativelyundesirable in a commercial application.

[0137] Embodiments of the invention, such as the multicarrierde-cresting circuit 1000 described in connection with FIG. 10,advantageously overcome the undesirable polluting of unoccupied channelallocations by injecting multiple band-limited pulses from multiplepulse generators. In one embodiment, the multiple band-limited pulsesare Gaussian pulses. The generation of multiple band-limited pulsesallows the pulse generator control to determine the PSD content in eachof the allocated channels and advantageously insert Gaussian pulseenergy only into occupied channels to counteract the signal peak. Thisadvantageously prevents the injection of Gaussian pulse energy tounoccupied channel allocation.

[0138] Further, one embodiment of the pulse generator control 1008 isprovided with the individual amplitude levels for each basebandchannel's contribution to the overall composite signal's peak, so thatthe pulse generator control 1008 can weigh the amplitude of eachGaussian pulse according to the contribution to the peak in thecomposite multicarrier signal 1010.

[0139] FIGS. 13A-E illustrate the operation of the pulse generatorcontrol 1008 described in connection with FIG. 10. The pulse generatorcontrol 1008 advantageously provides multiple band-limited pulses, suchas Gaussian pulses, that destructively interfere with the signal crestsin the composite multicarrier signal 1010. With reference to FIGS.13A-E, horizontal axes 1302, 1304, 1306, 1308, 1310 indicate time. Asshown in FIGS. 13A-E, time increases to the right.

[0140]FIG. 13A includes a first waveform 1312, which corresponds to aportion of the composite multicarrier signal 1010. The first waveform1312 further includes a waveform crest 1314, which corresponds to arelatively high-amplitude signal crest in the composite multicarriersignal 1010. The first waveform 1312 and the waveform crest 1314 aresimilar to the first waveform 1112 and the waveform crest 1114 describedin connection with FIG. 11A.

[0141]FIG. 13B illustrates a second waveform 1316 that includescancellation pulses 1318, 1320 that are generated from a family ofband-limited pulses 1322, 1324, 1326, 1328, 1330, such as Gaussianpulses. In contrast to a single destructive pulse, such as the pulse1118 described earlier in connection with FIG. 11B, the cancellationpulses 1318, 1320 in the second waveform 1316 include multiplecancellation pulses. The pulses in the family of band-limited pulses1322, 1324, 1326, 1328, 1330 are selected to be centered at thecorresponding active channel frequencies. The cancellation pulses 1318,1320 of the second waveform 1316 are generated by the de-cresting pulsegeneration circuit 1004 to destructively interfere with the relativelyhigh-amplitude signal crest in the composite multicarrier signal 1010 asillustrated by the waveform crest 1314.

[0142]FIG. 13C illustrates a third waveform 1332, which corresponds tothe time-delayed composite multicarrier signal 1016. The time delaycircuit 1012 delays the composite multicarrier signal 1010 to thetime-delayed composite multicarrier signal 1016 to compensate for thecomputational latency of the de-cresting pulse generation circuit 1004.This alignment is shown in FIGS. 13B and 13C by the alignment of adelayed signal crest 1334 with the cancellation pulses 1318, 1320.

[0143] The cancellation pulses 1318, 1320 destructively interfere withthe relatively high signal peak in the time-delayed compositemulticarrier signal 1016. FIG. 13D illustrates a fourth waveform 1336,which corresponds to the output of the multi-input summing junction1014. The fourth waveform 1336 is thus the linear superposition of thesecond waveform 1316 and the third waveform 1332. In the fourth waveform1336, a compensated portion 1338 is substantially devoid of the waveformcrest 1314 by the destructive interference induced by the band-limitedpulse. FIG. 13E superimposes the second waveform 1316, the thirdwaveform 1332, and the fourth waveform 1336.

[0144]FIGS. 14A and 14B illustrate the results of a complementaryfrequency domain analysis of the multicarrier de-cresting circuit 1000.With reference to FIG. 14A, a trace 1402 is a basic PSD plot of thecomposite multicarrier signal 1010, which is provided as an input to thede-cresting pulse generation circuit 1004. A trace 1404 is a PSD plot ofthe multiple Gaussian pulses, which are the outputs of the de-crestingpulse generation circuit 1004. A trace 1406 is a PSD plot of thede-crested composite multicarrier signal 1018 of the multi-input summingjunction 1014, which combines the time-delayed composite multicarriersignal 1016 with the multiple Gaussian pulses. The trace 1406illustrates that the PSD bandwidth expansion of the de-crested compositemulticarrier signal 1018 can be relatively readily controlled bymanaging the PSD of the corresponding multiple Gaussian pulses from thede-cresting pulse generation circuit 1004.

[0145] With reference to FIG. 14B, a trace 1408 is a PSD plot of thecomposite multicarrier signal 1010, where the composite multicarriersignal 1010 includes a non-utilized channel allocation. Advantageously,embodiments of the invention can inject multiple Gaussian pulses todestructively interfere with signal peaks at the utilized channelallocations, thereby preventing the expansion or pollution of thefrequency spectrum. A trace 1410 is a PSD plot of multiple Gaussianpulses, which correspond to output of the de-cresting pulse generationcircuit 1004. In one embodiment, each of the multiple Gaussian pulsesgenerated by the de-cresting pulse generator is substantiallyband-limited to its corresponding channel. A trace 1412 is a PSD plot ofthe de-crested composite multicarrier signal 1018 of the multi-inputsumming junction 1014, which combines the time-delayed compositemulticarrier signal 1016 with the multiple Gaussian pulses. In contrastto the injection of a single Gaussian pulse de-crest the compositemulticarrier signal 1010, which is illustrated in FIG. 12C, theinjection of multiple Gaussian pulses corresponding only to allocatedchannels is advantageously relatively free from spectral pollution.

[0146]FIG. 15 illustrates one embodiment of the de-cresting pulsegeneration circuit 1004. The de-cresting pulse generation circuit 1004advantageously provides multiple band-limited pulses to de-crest thecomposite multicarrier signal 1010 with relatively little pollution ofthe frequency spectrum.

[0147] The illustrated de-cresting pulse generation circuit 1004includes the pulse generator control 1008 and a pulse generator 1502.The pulse generator control 1008 shown in FIG. 15 further includes acomparator 1504, a weight generator 1506, and an impulse generator 1508.

[0148] The composite multicarrier signal 1010 is provided as an input tothe comparator 1504. In addition, the comparator 1504 receives channelinputs from the pulse shaping filters and phase information from digitalNCO sources. This information enables the comparator 1504 to determinewhether to apply single or multiple cancellation pulses to de-crest thecomposite multicarrier signal 1010 or the time-delayed compositemulticarrier signal 1016. In one embodiment, the comparator 1504compares these signals to reference information of the intrinsicwaveform. The reference information can include the average, the peak,and other pertinent signal statistics to determine whether to applycancellation pulses to de-crest the composite multicarrier signal 1010.

[0149] When the comparator 1504 has determined that a cancellation pulseor a group of cancellation pulses will be applied, the comparator 1504calculates a duration for a cancellation pulse and instructs the impulsegenerator 1508 to provide a sequence of impulses to the pulse generator1502.

[0150] The weight generator 1506 provides weight values to the pulsegenerator 1502. The weight values are used by the pulse generator 1502to vary an amount of a band-limited de-cresting pulse injected into achannel according to the weight value corresponding to the channel.

[0151] In one embodiment, the weight generator 1506 calculates arelative magnitude and phase for each channel's contribution to thecrest in the composite multicarrier signal 1010 and provides weightvalues to the pulse generator 1502 so that each channel suffers anapproximately equal degradation in signal quality. The weight valuesgenerated by the weight generator 1506 can advantageously be set at azero weight for inactive channels and a relatively high weight forrelatively high-power channels. The weight values can correspond topositive values, to negative values, to zero, and to complex values.This allows the error vector magnitude (EVM) to be approximately equalfor all active channels, while simultaneously eliminating or reducingsignal crests.

[0152] In another embodiment, a single active channel is randomlyselected for introduction of a stronger correction pulse. This lowersaggregate error rates, but increases the severity of the errors.

[0153] The pulse generator 1502 includes a group of multipliers 1510, agroup of filters 1512, and a summing circuit 1514. It will be understoodby one of ordinary skill in the art that the waveshaping circuits andsub-circuits disclosed herein can be configured to process an arbitraryor “N” number of channels. In addition, although the pulse generator1502 can include processing capability for several channels, it will beunderstood by one of ordinary skill in the art that some applicationswill not utilize all of the processing capability.

[0154] The group of multipliers 1510 in the illustrated pulse generator1502 can include “N” multipliers. A first multiplier 1516 multiplies theimpulses from the pulse generator 1502 with the weight value from theweight generator 1506 that corresponds to a first channel. A secondmultiplier 1518 similarly multiplies the impulses from the pulsegenerator 1502 with the weight value from the weight generator 1506 thatcorresponds to a second channel.

[0155] The group of filters 1512 in the illustrated pulse generator 1502can include “N” passband filters. A first passband filter 1520 generatesband-limited pulses in response to receiving impulses from the firstmultiplier 1516. The band-limited pulses from the first passband filter1520 are centered at approximately the first channel's frequency band orallocation. In one embodiment, the first passband filter 1520 is aGaussian passband finite impulse response (FIR) filter.

[0156] A second passband filter 1522 similarly generates band-limitedpulses in response to receiving impulses from the second multiplier1518. The band-limited pulses from the second passband filter 1522 arecentered at approximately the second channel's frequency band orallocation. In one embodiment, the second passband filter 1522 is aGaussian passband FIR filter. Preferably, all passband filters in thegroup of filters 1512 are FIR filters so that the outputs of thepassband filters are phase aligned.

[0157] The summing circuit 1514 combines the outputs of the firstpassband filter 1520, the second passband filter 1522, and otherpassband filters, as applicable, in the group of filters 1512. Theoutput of the summing circuit 1514 is a composite stream of Gaussianpulses, which is then applied to the multi-input summing junction 1014to reduce or to eliminate relatively high amplitude signal crests. Inanother embodiment, the individual outputs of the passband filters inthe group of filters 1512 are applied directly the multi-input summingjunction 1014.

[0158]FIG. 16 illustrates a multiple channel circuit 1600 according toan embodiment of the present invention. The multiple channel circuit1600 advantageously reduces the likelihood of the occurrences of signalcrests in composite waveforms, and can be used to decrease a frequencyof application of waveshaping. It will be understood by one of ordinaryskill in the art that the number of channels pulse shaped and combinedby the multiple channel circuit 1600 can be arbitrarily large.

[0159] The multiple channel circuit 1600 includes fractional delays,which stagger the input symbol streams relative to each other byfractions of a symbol period. In one embodiment, the delay offset fromone symbol stream to another is determined by allocating the symbolperiod over the number of active symbol streams. For example, where “x”corresponds to a symbol period and there are four input symbol streams,a first symbol stream can have 0 delay, a second input symbol stream canhave 0.25x delay, a third input symbol stream can have 0.50x delay, anda fourth input symbol stream can have 0.75x delay.

[0160] The illustrated embodiment of the multiple channel circuit 1600implements the fractional delay to the data streams before the pulseshaping filters. In one example, “N,” or the number of active symbolstreams, corresponds to 4. In the multiple channel circuit 1600, a firstinput symbol stream 1602 is applied as an input directly to a firstpulse-shaping filter 1604 without fractional delay. In anotherembodiment, the data stream associated with the first input symbolstream 1602 includes a fractional delay.

[0161] A second input symbol stream 1606 is provided as an input to afirst fractional delay circuit 1608, which delays the second inputsymbol stream 1606 relative to the first input symbol stream 1602 by afirst fraction of a symbol period, such as 0.25 of the symbol period. Athird input symbol stream 1612 is provided as an input to a secondfractional delay circuit 1614, which delays the third input symbolstream 1612 relative to the first input symbol stream 1602 by a secondfraction of the symbol period, such as 0.50 of the symbol period. Afourth input symbol stream 1618 is applied to a third fractional delaycircuit 1620, which delays the fourth input symbol stream 1618 by athird fraction of a symbol period, such as 0.75 of the symbol period.

[0162] The staggered symbol streams are mixed by their respective mixercircuits 1624, 1626, 1628, 1630 and combined by a summing circuit 1632.The staggering of the symbol streams reduces the probability ofoccurrence of signal crests in the resulting composite waveform 1634because the staggering displaces each channel's individual signal crestfrom another channel's signal crest as a function of time. Thisdecreases the probability of a mutual alignment in amplitude and phasein the composite waveform 1634.

[0163] However, it will be understood by one of ordinary skill in theart that the fractional delay can be applied elsewhere, such as embeddeddirectly within a pulse-shaping filter, applied post pulse-shaping, andthe like. In one embodiment, the amount of the fractional delay for eachsymbol stream is fixed in hardware. In another embodiment, thefractional delays can be selected or programmed by, for example,firmware.

[0164] Some systems that are susceptible to relatively high-amplitudesignal peaks or crests are incompatible with techniques that modify theamplitude of the underlying signals to reduce or to eliminate therelatively high-amplitude signal peaks in a composite multicarriersignal. One example of such a system is an EDGE system, whereintroduction of amplitude modulating pulses such as band-limitedGaussian pulses is undesirable and may not be permissible.

[0165]FIG. 17 illustrates a phase-modulating waveshaping circuit 1700according to an embodiment of the present invention. Advantageously, thephase-modulating waveshaping circuit 1700 reduces or eliminatesrelatively high-amplitude signal crests in composite multi-carriersignals without modulation of the amplitude of the underlying signals.Rather than sum a composite multicarrier signal with band-limited pulsesto de-crest the composite multicarrier signal as described in connectionwith FIG. 10, the phase-modulating waveshaping circuit 1700 modulatesthe phases of the input symbol streams to reduce or to eliminaterelatively high signal crests in the resulting composite multicarriersignal. It will be understood by one of ordinary skill in the art thatthe phase-modulating waveshaping circuit 1700 can be configured toprocess an arbitrary or “N” number of channels.

[0166] The phase-modulating waveshaping circuit 1700 includes a multiplechannel circuit 1702, a de-cresting combiner 1704, digital NCOs 1706,and a pulse phase modulation circuit 1708. The multiple channel circuit1702 receives the input symbol streams, pulse shapes and upconverts theinput symbol streams. The pulse shaped and upconverted input streams areprovided as inputs to the de-cresting combiner 1704 and to a pulse phasemodulator control 1710 of the pulse phase modulation circuit 1708.

[0167] One embodiment of the pulse phase modulation circuit 1708 issubstantially the same as the de-cresting pulse generation circuit 1004described in connection with FIGS. 10 and 15. However, rather thansumming the composite multicarrier signal with the generatedband-limited pulses, the band-limited pulses are used to phase modulatethe upconverted symbol streams. As such, the pulse phase modulatorcontrol 1710 corresponds to the pulse generator control 1008. The pulsephase modulator control 1710 predicts whether the current modulationstreams and digital NCO phase combinations will constructively interferewith each other and result in a composite waveform crest. Where a crestis predicted, the Gaussian pulse phase modulators are engaged torelatively slowly modulate the individual channel phases to prevent orto reduce a signal crest in the composite waveform.

[0168] A Gaussian pulse phase modulator, such as a first Gaussian pulsephase modulator 1712 corresponds to a Gaussian pulse generator, such asa first Gaussian pulse generator 1020. Again, the corresponding Gaussianpulses gp₁(t), gp₂(t), and so forth, generated by the Gaussian pulsephase modulators of the pulse phase modulation circuit 1708 areband-limited to their corresponding input symbol stream's allocatedchannel.

[0169] The de-cresting combiner 1704 includes multiple delay circuits1714, 1716, 1718, 1720, which align the upconverted symbol streams fromthe multiple channel circuit 1702 with the Gaussian pulses from thepulse phase modulation circuit 1708. The de-cresting combiner 1704further includes phase modulators 1722, 1724, 1726, 1728, which phasemodulate their respective upconverted input symbol streams in accordancewith the respective Gaussian pulse from the pulse phase modulationcircuit 1708. A summing circuit 1730 combines the outputs of the phasemodulators 1722, 1724, 1726, 1728 and provides a de-crested compositemulticarrier signal 1732 as an output.

[0170] The skilled practitioner will recognize that care should be takento ensure that the rate of change of phase due to this correctionprocess does not exceed the capability of the downstream receivers totrack effective channel phase variations.

[0171] One embodiment of the present invention further uses a pulsegenerator control or a pulse phase modulator control that is alreadyused to de-crest or to waveshape composite signals to continuallymonitor and to report the amplitude and phase information of eachindividual baseband channel. This information can be readily utilized toextract the average and peak power levels of individual channels. Inaddition, the presence of active or dormant channels can be readilyascertained. This information is extremely useful for externalsubsystems in a range of communications applications.

[0172] In one embodiment, a waveshaping circuit includes acommunications port, such as a serial communications port or a parallelcommunications port that enables this information to be transmitted toexternal devices. In another embodiment, the collected information isstored in a memory structure, which is accessed by multiple externaldevices requiring such information. The information can be ported to anamplifier linearization chip such as the PM7800 PALADIN product fromPMCS.

[0173] One embodiment of the waveshaping circuit is implemented indedicated hardware such as a field programmable gate array (FPGA) ordedicated silicon in an application specific integrated circuit (ASIC).In a relatively low data rate application, a general purpose digitalsignal processor (DSP), such as a TMS320C60 from Texas InstrumentsIncorporated or a SHARC processor from Analog Devices, Inc., performsthe waveshaping signal processing.

[0174] A conventional microprocessor/microcontroller or general purposeDSP can interface to a waveshaping circuit to adaptively control thewaveshaping process. For example, a de-cresting control can operate innon-real time, and a general purpose DSP or microprocessor such as aTMS320C54/TMS320C60/TMS320C40/ARM7 or Motorola 68000 device can be usedfor control. Preferably, the DSP or microprocessor includes non-volatileROM for both program storage and factory installed default parameters.Both ROM and Flash ROM are relatively well suited for this purpose. Aswith most DSP or microprocessor designs, a proportional amount of RAM isused for general-purpose program execution. In one embodiment, arelatively low speed portion of the waveshaping circuit implemented witha DSP or a microprocessor core and a relatively high speed portion ofthe waveshaping circuit implemented in an ASIC or an FPGA is integratedonto a single ASIC chip with an appropriate amount of RAM and ROM.Examples of licensable cores include the ARM7 from Advanced RISCMachines, Ltd., the Teak from DSP Group Inc., the Oak from DSP GroupInc., and the ARC from ARC Cores.

[0175] Various embodiments of the present invention have been describedabove. Although this invention has been described with reference tothese specific embodiments, the descriptions are intended to beillustrative of the invention and are not intended to be limiting.Various modifications and applications may occur to those skilled in theart without departing from the true spirit and scope of the invention asdefined in the appended claims.

What is claimed is:
 1. A waveshaping circuit that shapes a firstwaveform to decrease a ratio of peak power to average power in the firstwaveform such that an available power of a radio frequency poweramplifier can be efficiently used, where the shaping of the firstwaveform is substantially free from spectral pollution, the waveshapingcircuit comprising: a preconditioning circuit adapted to receive aninput symbol stream, the preconditioning circuit configured to comparedata in the input symbol stream to a first reference and to modify thedata in the input symbol stream by applying a first impulse to the inputsymbol stream selected to at least partially reduce the magnitude of asignal peak in the first waveform when the input symbol stream exceedsthe first reference, the preconditioning circuit further configured toprovide the modified symbol stream to a pulse-shaping filter, which mapsthe modified symbol stream to a baseband stream, where the pulse-shapingfilter is configured to provide the baseband stream to a mixer, whichupconverts the baseband stream by multiplication with an oscillatorsignal from a digital numerically controlled oscillator to anupconverted signal; a pulse generator adapted to receive the upconvertedsignal and to receive phase information from the digital numericallycontrolled oscillator, the pulse generator configured to generate aband-limited pulse when the pulse generator detects that the upconvertedsignal has a signal crest above a predetermined threshold, where theband-limited pulse is substantially limited to a frequency bandallocated to the input symbol stream; a delay circuit configured todelay the upconverted signal to a delayed upconverted signal, where anamount of delay is approximately equal to a latency in the pulsegenerator; and a summing circuit adapted to sum the band-limited pulsefrom the pulse generator with the delayed upconverted signal from thedelay circuit to generate the first waveform.
 2. The waveshaping circuitas defined in claim 1, wherein the band-limited pulse is selected fromthe group consisting of a band-limited Gaussian pulse, a band-limitedSquare Root Raised Cosine (SRRC) pulse, a band-limited Raised Cosine(RC) pulse, and a band-limited Sinc pulse.
 3. The waveshaping circuit asdefined in claim 1, wherein the pulse generator further comprises: acomparator adapted to detect when the upconverted baseband stream has asignal crest above the predetermined threshold; an impulse generatoradapted to provide a second impulse in response to an indication fromthe comparator that the signal crest is above the predeterminedthreshold; a weight generator that provides a weight value to scale amagnitude of the second impulse; and a finite impulse response (FIR)filter configured to filter the second impulse to substantially limitthe second impulse to a frequency band allocated to the input symbolstream.
 4. The waveshaping circuit as defined in claim 1, wherein thepreconditioning circuit is configured to receive updates to vary thefirst reference, and where the pulse generator is configured to receiveupdates to vary the predetermined threshold.
 5. The waveshaping circuitas defined in claim 1, where the input symbol stream comprises aplurality of input symbol streams, which are processed by thepreconditioning circuit, pulse-shaped, upconverted and combined to theupconverted baseband stream, and where the band-limited pulse generatedby the pulse generator is substantially limited to a frequency bandallocated to an input symbol stream from the plurality of input symbolstreams.
 6. The waveshaping circuit as defined in claim 5, furthercomprising a predictive weight generator circuit coupled to theplurality of input symbol streams and to a plurality of digitalnumerically controlled oscillators that are used to upconvertpulse-shaped input symbol streams, the predictive weight generatorcircuit configured to at least partially disable the preconditioningcircuit and the pulse generator in real time in response to a predictionthat the plurality of input symbol streams at least partiallydestructively interfere in the combined upconverted baseband stream. 7.The waveshaping circuit as defined in claim 1, where the input symbolstream comprises a plurality of input symbol streams, which areprocessed by the preconditioning circuit, pulse-shaped, upconverted andcombined to the upconverted baseband stream, and where the band-limitedpulse generated by the pulse generator comprises a plurality ofband-limited pulses, which are applied to the combined upconvertedbaseband stream to destructively interfere with the signal peak, whereeach of the plurality of band-limited pulses corresponds to a frequencyband allocated to an input symbol stream from the plurality of inputsymbol streams.
 8. An adaptive control circuit that provides parameterupdates to a digital waveshaping circuit, the adaptive control circuitcoupled to at least one input symbol stream and to an output samplestream, where the input symbol stream is also provided as an input tothe digital waveshaping circuit and the output sample stream isgenerated by the digital waveshaping circuit, the adaptive controlcircuit comprising: a reference input adapted to receive referenceinformation, where the parameter updates are controlled at least in partby data received at the reference input; an input monitoring circuitadapted to monitor the at least one input symbol stream provided to thedigital waveshaping circuit; a receiver circuit adapted to monitor theoutput sample stream from the digital waveshaping circuit; and aparameter update circuit adapted to calculate and to provide updatedparameters to the digital waveshaping circuit based on the referenceinput, a monitored portion of the at least one input symbol stream, andthe output sample stream.
 9. The adaptive control circuit as defined inclaim 8, wherein the parameter update circuit calculates the updatedparameters in non real time.
 10. A preconditioning circuit adapted toreduce an amplitude of a signal peak in an input symbol stream in realtime, where an output of the preconditioning circuit is applied to apulse-shaping filter, the preconditioning circuit comprising: acomparator coupled to the input symbol stream to compares a symbol fromthe input symbol stream to a reference level, the comparator configuredto generate a correction vector in response to the symbol exceeding thereference level; a pseudo random sequence generator adapted to generatea pseudo random noise sequence; a weight generator coupled to thecomparator and to the pseudo random sequence generator, the weightgenerator configured to provide a weight factor based on the correctionvector and the received pseudo random noise sequence; a first delaycircuit coupled to the pseudo random sequence generator, where the firstdelay circuit is adapted to delay an impulse from the pseudo randomsequence generator by a time approximately equal to a latency in theweight generator; a multiplier circuit coupled to the first delaycircuit and to the weight generator, where the multiplier circuit isconfigured to multiply an impulse from the first delay circuit with acorresponding weight factor from the weight generator in order to selectthe impulse from the pseudo random noise sequence and to scale theselected impulse; a second delay circuit coupled to the input symbolstream, where the second delay circuit is configured to delay the inputsymbol stream by a time approximately equal to a latency in thecomparator, the weight generator, and the multiplier circuit; and asumming circuit coupled to the multiplier circuit and to the seconddelay circuit, where the summing circuit is configured to sum theimpulse selected and scaled by the multiplier circuit with the inputsymbol stream from the second delay circuit to generate the output ofthe preconditioning circuit.
 11. The preconditioning circuit as definedin claim 10, wherein a value of the weight factor generated by theweight generator includes positive values, negative values, zero, andcomplex numbers.
 12. The preconditioning circuit as defined in claim 10,wherein the comparator is configured to receive adaptive updates to thereference.
 13. The preconditioning circuit as defined in claim 10,wherein the preconditioning circuit is adapted to at least decrease anamount of the impulse selected and scaled by the multiplier circuit inresponse to a prediction of destructive interference to a signal peak ina composite waveform that includes pulse-shaped and upconverted from theinput symbol stream.
 14. The preconditioning circuit as defined in claim10, wherein the preconditioning circuit is adapted to at least decreasean amount of the impulse selected and scaled by the multiplier circuitin response to a prediction of destructive interference to a signal peakin a composite waveform that includes pulse-shaped and upconverted fromthe input symbol stream.
 15. The preconditioning circuit as defined inclaim 10, wherein the comparator further comprises: a magnitudecomputation circuit adapted to compute a magnitude of the input symbolstream; a Taylor series approximation circuit adapted to compute aTaylor series expansion of a nonlinear weighting function with thecomputed magnitude as an input; a delay circuit adapted to delay theinput symbol stream by a time substantially equal to latency in themagnitude computation circuit and the FIR filter; and a mixer adapted tomultiply a filtered output of the FIR filter with the delayed inputsymbol stream from the delay circuit.
 16. A digital waveshaping circuitthat decreases an amplitude of a selected portion of a compositemulticarrier signal in real time, where the composite multicarriersignal includes a plurality of input symbol streams that have beenpulse-shaped and frequency up-converted, where the decrease in amplitudeof the selected portion allows a power capability of a related radiofrequency amplifier to be more efficiently used, the digital waveshapingcircuit comprising: means for monitoring the plurality of input symbolstreams that eventually combine to the composite multicarrier signal;means for monitoring phases of a plurality of carriers from a pluralityof digital numerically controlled oscillators (NCOs), where theplurality of oscillator signals are mixed with a plurality ofpulse-shaped input signal streams to upconvert the plurality ofpulse-shaped input signal streams; means for monitoring the compositemulticarrier signal to identify a signal peak above a selectedthreshold; means for determining a first symbol stream's contribution tothe detected signal peak in the composite multicarrier signal; means forgenerating at least a first band-limited pulse selected to destructivelyinterfere with at least a portion of the identified signal peak, wherethe first band-limited pulse is substantially limited to a frequencyband allocated to the first symbol stream; and means for combining thecomposite multicarrier signal with the at least one band-limited pulseto reduce the signal peak.
 17. A method of shaping a first waveform todecrease a ratio of peak power to average power in the first waveform bydigitally modifying data in a data stream that gives rise to the firstwaveform, where the shaping of the first waveform is substantially freefrom spectral pollution, the method comprising: comparing data in aninput symbol stream to a first reference; modifying the input symbolstream by applying an impulse to the input symbol stream selected to atleast partially reduce the magnitude of a signal peak in the firstwaveform when the comparing indicates that a corresponding data in theinput symbol stream exceeds the first reference; providing the modifiedinput symbol stream to a pulse-shaping filter, which maps the modifiedinput symbol stream to a baseband stream, where the baseband stream isupconverted by a mixer and a carrier information stream to anupconverted stream; comparing the upconverted stream to a secondreference; and generating the first waveform by applying a band-limitedpulse to the upconverted stream selected to at least partially reducethe signal peak in the upconverted stream when the comparing indicatesthat at least a portion of the upconverted stream exceeds the secondreference, where the band-limited pulse is substantially limited to afrequency band allocated to the input symbol stream.
 18. The method asdefined in claim 17, wherein the band-limited pulse is a band-limitedGaussian pulse.
 19. The method as defined in claim 17, wherein theimpulse applied to the input symbol stream is selected by applyingweight values to a sequence of pulses from a pseudo-random sequencegenerator.
 20. The method as defined in claim 17, wherein a size of theimpulse applied to the input symbol stream and a size of theband-limited pulse applied to the upconverted stream are under adaptivecontrol.
 21. The method as defined in claim 17, wherein the input symbolstream is an input symbol stream in a plurality of input symbol streams,where the plurality of input symbol streams are pulse-shaped to aplurality of baseband streams, where the plurality of baseband streamsare upconverted by multiple carriers to upconverted streams, where theupconverted streams are combined to a composite stream, where at leastone band-limited pulse is applied to the composite stream todestructively interfere with the signal peak, where the at least oneband-limited pulse corresponds to a frequency band allocated to an inputsymbol stream from the plurality of input symbol streams.
 22. The methodas defined in claim 21, wherein the at least one band-limited pulsecomprises a plurality of band-limited pulses applied to the compositestream to destructively interfere with the signal peak, where each ofthe plurality of band-limited pulses corresponds to a frequency bandallocated to an input symbol stream from the plurality of input symbolstreams.
 23. The method as defined in claim 21, further comprising:predicting a level of the composite stream; comparing the predictedlevel of the composite stream to a predetermined threshold; reducing, inreal time, an amount of the impulse applied to the input symbol streamwhen the predicted level of the composite stream is below thepredetermined threshold; and reducing, in real time, an amount of the atleast one band-limited pulse applied to the composite stream.
 24. Amethod of adaptively controlling a digital waveshaping process, themethod comprising: receiving a reference information as a control input;monitoring at least one input symbol stream applied to the waveshapingprocess; monitoring an output of the waveshaping process, where theoutput includes a waveform that is pulse-shaped and upconverted from theat least one input symbol stream; updating a first parameter used toselect an impulse that is applied to the input symbol stream to at leastpartially reduce the magnitude of a signal peak in the output of thewaveshaping process; and updating a second parameter used to select aband-limited pulse that is applied to the output of the waveshapingprocess.
 25. The method as defined in claim 24, wherein the method isnot performed in real time.
 26. The method as defined in claim 24,wherein the reference information indicates is applied as a controlparameter to a non-linear function that controls a hardness of limiting,the method further comprising calculating the first parameter and thesecond parameter to achieve the limiting as specified by the non-linearfunction.
 27. The method as defined in claim 24, wherein the at leastone input stream comprises a plurality of input streams, which arepulse-shaped, upconverted, combined, and waveshaped to generate theoutput.
 28. A method of digitally preconditioning an input symbol streamto a pulse-shaping filter in real time, the method comprising: comparinga symbol from the input symbol stream to a reference; generating aweight value in response to the comparison between the symbol and thereference; receiving a pseudo random sequence; multiplying an impulsefrom the pseudo random sequence with the weight value to generate acorrection impulse; and summing the correction impulse with the symbol.29. The method as defined in claim 28, wherein the weight value includeszero.
 30. The method as defined in claim 28, wherein the weight valuecan take on positive values, negative values, zero, and complex numbers.31. The method as defined in claim 28, further comprising adaptivelyupdating the reference.
 32. The method as defined in claim 28, whereinthe input symbol stream comprises a plurality of input symbol streamsand the preconditioning is applied to the plurality of input symbolstreams, the method further comprising decreasing a magnitude of thecorrection impulse in response to a prediction that a peak level of acomposite signal is less than a predetermined threshold, where thecomposite signal is generated by pulse-shaping, frequency upconverting,and combining of the plurality of input symbol streams, and where thepredicted peak level is computed without preconditioning applied to theplurality of input symbol streams.
 33. The method as defined in claim28, wherein the input symbol stream comprises a plurality of inputsymbol streams and the preconditioning is applied to the plurality ofinput symbol streams, the method further comprising disablingapplication of the correction impulse in response to a prediction that apeak level of a composite signal is less than a predetermined threshold,where the composite signal is generated by pulse-shaping, frequencyupconverting, and combining of the plurality of input symbol streams,and where the predicted peak level is computed without preconditioningapplied to the plurality of input symbol streams.
 34. The method asdefined in claim 28, further comprising: computing a magnitude of datain the input symbol stream; applying the magnitude to a Taylor seriesexpansion of a nonlinear weighting function; and multiplying a result ofthe Taylor series expansion with the data in the input symbol stream toprovide the preconditioning to the input symbol stream.